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Parts List for Multicell Robotic Eye ADC0816 eight-bit analog-to-digital converter IC 2.2K resistor (adjust value to gain best response of photocells) 2.2K resistors Photocell

Simple lenses and filters can be used to greatly enhance the sensitivity, directionality, and effectiveness of both single- and multicell-vision systems. By placing a lens over a small cluster of light cells, for example, you can concentrate room light to make the cells more sensitive to the movement of humans and other animate objects. The lens need not be complex; an ordinary 1 2- to 1-in-diameter magnifying lens, purchased new or surplus, is all you need. You can also use optical filters to enhance the operation of light cells. Optical filters work by allowing only certain wavelengths of light to pass through and blocking the others. CdS photocells tend to be sensitive to a wide range of visible and infrared light. You can readily accentuate the sensitivity of a certain color (and thereby de-accentuate other colors) just by putting a colored gel or other filter over the photocell.

Lenses are refractive media constructed so that light bends in a particular way. The two most important factors in selecting a lens for a given application are lens focal length and lens diameter:

Lens focal length. Simply stated, the focal length of a lens is the distance between the lens and the spot where rays are brought to a common point. (Actually, this is true of positive lenses only; negative lenses behave in an almost opposite way, as discussed later.) Lens diameter. The diameter of the lens determines its light-gathering capability. The larger the lens is, the more light it collects.

There are six major types of lenses, shown in Fig. 32-8. Such combinations as planoconvex and bi-concave refer to each side of the lens. A plano-convex lens is flat on one side

FIGURE 32-8 Lenses come in a variety of forms. Plano-convex and double-convex are among the most common.

and curved outward on the other. A bi-concave lens curves inward on both sides. Negative and positive refer to the focal point of the lens, as determined by its design. Lenses form two kinds of images: real and virtual. A real image is one that is focused to a point in front of the lens, such as the image of the sun focused to a small disc on a piece of paper. A virtual image is one that doesn t come to a discrete focus. You see a virtual image behind the lens, as when you are using a lens as a magnifying glass. Positive lenses, which magnify the size of an object, create both real and virtual images. Their focal length is stated as a positive number, such as +1 or +2.5. Negative lenses, which reduce the size of an object, create only virtual images. Their focal length is stated as a negative number. Lenses are common finds in surplus stores, and you may not have precise control over what you get. For robotics vision applications, plano-convex or double-convex lenses of about 0.5 to 1.25 in diameter are ideal. The focal length should be fairly short, about 1 to 3 in. When you are buying an assortment of lenses, the diameter and focal length of each lens is usually provided, but if not, use a tape to measure the diameter of the lens and its focal length (see Fig. 32-9). Use any point source except the sun focusing the light of the sun onto a small point can cause a fire! To use the lens, position it over the light cell(s) using any convenient mounting technique. One approach is to glue the lens to a plastic or wood lens board. Or, if the lens is the correct diameter, you can mount it inside a short length of plastic PVC pipe; attach the other end of the pipe to the light cells. Be sure you block out stray light. You can use black construction paper to create light baffles. This will make the robot see only the light shining through the lens. If desired, attach a filter over the light cells. You can use a dab of glue to secure the filter in place. Using Fig. 32-6 as a guide, you can create a kind of two-eyed robot by placing a lens over each group of four photocells. The lenses are mounted in front of the photocells,

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Parallel Rays (from Sun or Other Distant Point Source)